Currently, most Metropolitan Area Networks (MAN) and Wide Area Networks (WAN) are Time Division Multiplexing (TDM) Synchronous Digital Hierarchy (SDH) or Synchronous Optical Networks (SONET) based. In Local Area Networks (LAN) most networks are Ethernet based.
SDH/SONET (Synchronous Digital Hierarchy/Synchronous Optical Network) standards evolved originally for use in a voice network. SDH is a European version of a standard that is substantially the same as the SONET standard developed in North America. SDH/SONET contains connection oriented synchronous TDM circuit switching technology. The SDH/SONET configured network runs at the same clock domain (e.g., every section of the network can be traced to a primary clock reference). The network allocates fixed bandwidth time slots for each circuit. The SDH/SONET architectures are connection based protocols in that there is a physical circuit arrangement between ports in a switch to establish an end to end path. The digital transitions in signals traveling through an SDH/SONET network occur at the same rate, however there may be a phase difference between the transitions of any two signals caused by time delays or jitter in the transmission system.
Ethernet evolved primarily as a data network. In contrast to SDH/SONET, Ethernet is a connectionless asynchronous Carrier Sense, Multiple Access with Collision Detection (CSMA/CD) packet switching technology. The Ethernet architecture does not rely on a single clock domain like the SDH/SONET architecture. The Ethernet architecture sends a series of packets across the network containing data. Whenever a packet needs to be sent, the transmitter will try to transmit the packet. The Ethernet architecture is also connectionless in that the packets travel from node to node within the network without establishing a logical or physical circuit. The end to end path is discovered through a process called “bridging”. Ethernet is fundamentally a Local Area Network (LAN) technology.
SDH/SONET networks provide reliable, guaranteed available bandwidth, low jitter connections. These characteristics are required for voice quality networks. SDH/SONET, however, is bandwidth inefficient and has a higher overhead than many other network architectures. Ethernet networks, in contrast, provide lower reliability best effort delivery, and low cost bandwidth connections. These characteristics are suitable for data quality networks. Ethernet networks have non-guaranteed transmission and low overhead, and support fewer operational functions than SDH/SONET. In SDH/SONET, once the circuit is established, bandwidth is allocated for an application and cannot be used by any other application, even if the original application is not using the bandwidth. In Ethernet, applications only use bandwidth when they need the bandwidth to transmit packets.
In order to appreciate operation of the illustrative embodiments described herein, it is helpful to understand the Open Systems Interconnect (OSI) network hierarchy, which views a network as being composed of several hierarchical layers. In the hierarchy, Layer 1 is the physical layer containing elements that perform the transmission of signals within the network. Layer 2 is the data link layer, which provides services that allow direct communication between devices across the underlying physical channel of Layer 1. Layer 3 is the network layer, which is responsible for station-to-station data delivery over multiple data links. The network layer is responsible for the routing of packets across the network. Layer 4 is the transport layer, which provides an error-free, sequenced, guaranteed delivery, message service that allows process to process communication between stations on a network. Layer 5 is the session layer, which deals with the establishment of communications between applications. This layer is useful for security applications. Layer 6 is the presentation layer, which enables the sharing of data between networked systems using different methods of local data representation. Finally, Layer 7 is the application layer. This layer provides generic application functions, such as email, file transfer capability, and the like.
Currently, Ethernet networks do not support the concept of establishing circuits. Ethernet networks use bridging algorithms to flood and learn how to forward an Ethernet frame from a source station to a destination station. STP is a protocol wherein bridges determine, establish, and maintain a topology without loops that includes every reachable link in a catenet (a collection of networks interconnected at the data link layer). In contrast, SDH/SONET networks use manual or automated provisioning with various routing and signaling protocols and algorithms in higher layers to establish circuits between two TDM endpoints.
In the LAN/Ethernet data networking world, several features have evolved to simplify network management. Many of these features can be beneficial in the LAN environment, but have disadvantages in the WAN environment. A common feature is plug-and-play, i.e., the automatic configuration of components when connected to a system or network. Spanning Tree Protocol (STP) breaks all loops in the network by only allowing one path between stations. This means that all other links are inactive and hence bandwidth is wasted. Bandwidth is also relatively plentiful in LAN/Ethernet networks. An Ethernet frame from a source station to an unknown destination station is forwarded by flooding, which is also a waste of bandwidth. Again, bandwidth is relatively less costly and more plentiful in LANs. Restoration times for equipment/link failure are relatively long, i.e., on the order of seconds. The assumption in data networking is that in the transmission of data, applications are not as time sensitive as in voice networking and the upper layers will take care of recovery by re-transmitting. This means that fast recovery is not essential for data networking.
In the MAN/WAN/SDH/SONET world the opposite is true. Users demand full control of their network and hence want to provision all circuits using Signaling algorithms. Bandwidth is not plentiful, and is costly, in MAN/WAN. Therefore, all of the bandwidth for each link should be utilized as completely as possible. Routing algorithms discover or learn the path from a source station to a destination station. The Layer 3 routing algorithms are relatively more bandwidth efficient than the flooding approach of the Layer 2 Ethernet MAC bridging algorithm. Restoration time for equipment failure/line recovery in the SDH/SONET world is specified to be quick, i.e., less than 50 msec. In the voice world, i.e, SDH/SONET, applications are highly time sensitive and, hence, lower layers perform failure recovery of equipment/line because they have a faster react time.